1. Field of the Invention
This invention pertains generally to electrical switching apparatus and, more particularly, to circuit breakers including a limiter having a trip indicator member. The invention also relates to limiters and to trip indicators therefore.
2. Background Information
Current limiting power interruption requires a current interruption device that rapidly and effectively brings the current to a relatively low or zero value upon the occurrence of a line fault or overload condition.
Circuit protection devices protect electrical equipment from damage when excess current flows in the circuit due to overload or short circuit conditions. Such devices have a relatively low resistivity and, accordingly, high conductivity under normal current conditions of the circuit, but are “tripped” or converted to high or complete resistivity when excessive current and/or temperature occurs. When the device is tripped, a reduced or zero current is allowed to pass in the circuit, thereby protecting the wires and load from electrical and thermal damage until the overload or fault is removed.
Conventional circuit protection or current limiting devices include, but are not limited to, circuit breakers, fuses (e.g., expulsion fuses), thermistors (e.g., PTC (Positive Temperature Coefficient) conductive polymer thermistors), and the like. These devices are current rated for the maximum current the device can carry without interruption under a load.
Circuit breakers typically contain a load sensing element (e.g., a bimetal, hot-wire or magnetic element) and a switch which opens under overload or short circuit conditions. Most circuit breakers have to be reset manually at the breaker site or via a remote switch.
Fuses typically contain a load sensing fusible element (e.g., metal wire), which when exposed to current of fault magnitude rapidly melts and vaporizes through resistive heating (I2R). Formation of an arc in the fuse, in series with the load, can introduce arc resistance into the circuit to reduce the peak let-through current to a value significantly lower than the fault current. Expulsion fuses may further contain gas-evolving or arc-quenching materials which rapidly quench the arc upon fusing to eliminate current conduction. Fuses generally are not reusable and must be replaced after overload or short circuit conditions because they are damaged inherently, when the circuit opens.
Various fusible elements, gas-evolving materials and fuses are shown for example in U.S. Pat. Nos. 2,526,448; 3,242,291; 3,582,586; 3,761,660; 3,925,745; 4,008,452; 4,035,755; 4,099,153; 4,166,266; 4,167,723; 4,179,677; 4,251,699; 4,307,368; 4,309,684; 4,319,212; 4,339,742; 4,340,790; 4,444,671; 4,520,337; 4,625,195; 4,638,283; 4,778,958; 4,808,963; 4,950,852; 4,952,900; 4,975,551; 4,995,886; and 5,471,185.
Low-voltage circuit-breakers are often connected in series with so-called limiters, in order to significantly increase the short-circuit switching capacity in low-voltage electrical networks and to significantly limit the cut-off currents. Such limiters are designed to transition rapidly, in case of a short circuit, from a low-resistance state to a high-resistance state and contribute with their voltage requirement to rapid current limiting and disconnection. Some limiters employ, for example, fuses, such as fusible wire elements to accomplish this function.
It is known to provide a trip indication for a circuit breaker limiter through a button visible at the top surface thereof. The button changes position responsive to a spring when the button is released by a fused wire element. For example, the button “pops” above the surface of the limiter when the fused wire element burns opens during a fault.
As shown in FIG. 1, a molded button 2 includes a central opening 4 having a separate molded part 6 therein. The separate molded part 6 cooperates with a fused wire element 8 to retain the wire element 8 and to hold the button 2 in place against the bias of spring 10. The fused wire element 8 passes through an assembly frame 12 and gaskets 13 of a limiter 14 and is electrically connected in series between a circuit breaker load terminal (not shown) and a load (not shown). The central opening 4 in the molded button 2 permits the assembly of the wire element 8 and the separate molded part 6 outside of the top (with respect to FIG. 1) of the button 2. The bias spring 10 pushes the button 2 up when the wire element 8 burns open.
There is room for improvement in electrical switching apparatus and in limiters for such apparatus. There is also room for improvement in indicators for limiters.